This invention relates to a method of and apparatus for providing information as to the existence or position of objects otherwise than by direct vision.
More specifically a method to which the invention relates (hereinafter referred to as being of the kind specified) comprises the steps a radiating stress wave energy (herein called the transmitted signal) from a station to a field of view, receiving at the station at least part of the transmitted signal (such part herein being called the received signal) reflected from the object, carrying out a frequency modulation of the transmitted signal to produce a frequency difference between the received signal and a correspondingly frequency modulated locally generated signal in a mode such that the magnitude of the frequency difference represents the range of the object from the station, operating upon the received signal with the locally generated signal to produce an indicator signal of said difference frequency providing for range indication, and determining or sensing the angular position of the object with respect to a reference axis extending through the station and the field of view by observation of a characteristic of the indicator signal which is dependent upon the angular position of the object.
The apparatus to which the invention relates (hereinafter referred to as being of the kind specified) comprises a transmitting means for transmitting frequency modulated stress wave energy (herein called the transmitted signal) from a station to a field of view, receiving means at the station for receiving at least part of the wave energy (this part herein being called the received signal) reflected from an object in the field of view, modulator means for producing a frequency difference between the received signal and a correspondingly frequency modulated locally generated signal, said frequency modulation being in a mode such that the magnitude of the frequency difference is representative of the range of the object from the station, means for providing the locally generated signal and for feeding same to said modulator means to produce therefrom an indicator signal of said difference frequency, and indicator means responsive to said indicator signal to furnish observable range and direction information.
In both performance of the method and in the apparatus it is preferred that the stress wave energy shall be radiated at a frequency above audibility i.e. supersonic.
One form of apparatus of the kind specified for performing the method of the kind specified is disclosed in earlier developments of the present inventor published in U.S. Pat. No. 3,172,075 and 3,366,922 relating to a blind aid. In the latter, two receiving transducers on a head-set provide receptivities represented by divergent medially overlapping polar lobes, and signals received by these transducers are fed through channels in the receiving means providing audible difference frequency indicator signals to the left hand and right hand audio-transducers on the head-set. The polar lobe characteristics are designed to match the characteristics of the human auditory neural system so that a blind person wearing the head-set can sense the direction from which a reflection of the transmitted signal is received, and, of course, can also make a subjective determination of the range by virtue of the frequency of the indicator signal.
It has been recognized by the present inventor that radiation of very wide bandwidth frequency modulated waves, as disclosed in these earlier developments, provides an indicator signal which not only indicates distance and direction but carries information about the nature of the object. This information is in a form of tone complexes caused by multiple scattering, or by reflecting discontinuities of the object, and which arise from object shape, size, surface structure and in some cases from volume structure. For example an object in which volume structure is important is a shoal of fish. When the object or parts of the object move, the instantaneous velocity of each reflecting part resolved along the direction transmission of the stress wave, causes the re-radiated stress wave to change in frequency due to DOPPLER effect. This frequency change is transferred to the indicator signal, which is in the audible range, unaffected as regards the magnitude of the frequency change so that a small percentage change of the preferred supersonic radiation frequency will appear in the indicator signal as a significant and noticeable frequency change even though the acutal positional change of the object may be slight. Such variation in the components of a tone complex, as well as the form of the tone complex itself characterise the object producing this complex and enable a skilled observer to recognize character of the object.
Surfaces of an object, which have a small separation along the direction of propogation can be observed using a pulse echo location system only if the pulse durations are of the order of half the transit time in the medium between the surfaces at their minimum separation and this entails a very wide bandwidth system. Further, the time interval between radiated pulses would have to be greater than the transit time in the medium for objects at maximum range and would typically be of the order of one second for underwater applications. Change of position cannot be determined by a single echo, and the repetition frequency would be too low for the system to sample significant changes in position of the object.
The prior forms of apparatus referred make use of audible indicator means, indeed essentially so, where the characteristic of the human auditory neural system is to be simulated. There are, however, applications of methods and apparatus of the kind specified where other forms of indicator means may be advantageously employed, for example visual indicator means or tactile indicator means both providing sensually perceptible indications of range and direction. The purpose of the present invention, is to provide an improved method and apparatus by means of which object character recognition is enhanced when utilizing an aural indicator means, and which solves certain problems (more conveniently explained hereafter) which are entailed when visual or tactile indicator means are employed.